This disclosure relates generally to methods for manufacturing composite structures and, more particularly, to methods for forming composite articles made of fiber-reinforced plastic material (e.g., pre-preg).
As used herein, the term “pre-preg” refers to a reinforcement material that has been impregnated with either a thermoplastic or thermoset polymeric matrix material, such as resin. The term “thermoset pre-preg” will be used to refer to reinforcement material that has been impregnated with thermoset resin. Pre-preg allows one to impregnate the fibers on a flat workable surface, or rather in an industrial process, and then later form the impregnated fibers to a shape which could prove to be problematic for the hot injection process. The thermoset pre-preg is only partially cured to allow easy handling; this B-stage material requires cold (below 20° C.) storage to prevent further curing. The pre-preg can be stored in a cooled area for an extended period of time to cure later.
Traditional methods of manufacturing thermoset composite articles include laying up composite plies of thermoset pre-preg over a forming die or tool. Heat and pressure are then applied to the composite layup to consolidate and cure the composite layup. In some traditional fabrication methods, the consolidation and curing of composite layups must be performed inside of an autoclave to provide the compaction pressure required to achieve the necessary mechanical properties for the cured composite article.
Traditional autoclave processing of thermoset composite materials may require extended periods of time during which heat and pressure are applied to a composite layup on a forming tool. For example, the consolidation and curing of a composite layup in an autoclave can take up to 24 hours. Unfortunately, autoclaves are generally expensive to construct and operate. Furthermore, the forming tools or dies over which the composite parts are laid up are relatively expensive to manufacture and maintain. For production programs requiring a high volume of thermoset composite parts, a large number of forming tools may be required. The combination of expensive forming tools and autoclave processing translates into an overall high cost of production.
The foregoing drawbacks are compounded by the fact that composites materials are capable of high degrees of optimization. Because individual plies of pre-preg used for primary structure are very thin (e.g., <0.01 inch), very complex stacking sequences are often developed for design to optimize strength and weight. However, high degrees of design complexity drive high manufacturing cost. Large/expensive automated layup equipment is needed because complex designs are created with thin material forms. In order to build up laminates fast with thin material, the machines need to be large and accurate. State-of-the-art high-performance thermoset pre-pregs also require large expensive autoclaves to consolidate and cure with high quality. Because of the nature of the materials used and the manufacturing process, many large expensive rate tools are often necessary to meet production rates for wing and fuselage structure.
In particular, the above-described problems are prevalent in aircraft manufacturing. Current composite wing and fuselage structures use very thin thermoset pre-preg material that requires very expensive automated fiber placement equipment. Moreover, the process of placing ply upon ply using a fiber placement machine takes days to build up a structure having a desired thickness and strength.
Improved methods for manufacturing thermoset composite parts that allow for high production rates with reduced manufacturing costs would be desirable.